Some have argued that...fully 75% of all IQ variances can be attributed to genetic differences. Studies have shown this to be high, except in monozygotic (identical or MZ) twins raised together. These twins share the same exact genotype. In this case, the correlation was as high as .88. In the study of MZ twins raised apart, the correlation was as high as .75. In contrast, Dizygotic twins, who share 50% of their genes on average, had a correlation factor of .53 when growing up together and .46 when raised separately. This seems to indicate that similarity of a genetic component has a direct influence on IQ scores.

In The Bell Curve (1994), Richard Herrnstein and Charles Murray claim that separated-twin studies represent the "purest" of the direct measures of heritability and that intelligence is strongly heritable with a heritability estimate of 0.60+ 0.2 within whites.

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Another source of data: Nature vs Nurture in Intelligence

From http://www.wilderdom.com/personality/L4-1IntelligenceNatureVsNurture.html

A large number of the study of twins reared apart was undertaken by Thomas Bouchard of the University of Minnesota starting in 1979. He "collected" pairs of separated twins from all over the world and reunited them while testing their personalities and IQs. Other studies at the same time concentrated on comparing the IQs of adopted people with those of their adopted parents and their biological parents or their siblings. Put all these studies together, which include the IQ tests of tens of thousands of individuals, and the table looks like this:

Same person tested twice 87%

Identical twins reared together 86%

Identical twins reared apart 76%

Fraternal twins reared together 55%

Biological siblings reared together 47% (studies show that reared apart about 24%)

Parents and children living together 40%

Parents and children living apart 31%

Adopted children living together 0%

Unrelated people living apart 0%

Sloan observes: One could use the above data to argue that environment was as little as 10% responsible for IQ.. Note that 13% variation is simply due to retaking an IQ test. On the genetic influence on intelligence, I feel that Herrnstein and Murray's conclusion of 60 - 80% is supported by data on twins. Since IQ is fixed for life by around age five, environmental influences on IQ have a limited time span.

Scientists studied academic performance in more than 11,000 identical and non-identical 16-year-old twins in the UK.

The team from King's College London found that on average, genes explained 58% of differences between GCSE scores in core subjects such as maths.

Differences in grades due to environment, such as schools and families, accounted for about 36%.

The remaining differences in GCSE scores in maths, English and science are explained by environmental factors unique to each person, say the researchers.

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IQ AND ECONOMIC SUCCESS, by Charles Murray

excerpts fromhttp://www.cpsimoes.net/artigos/art_iq_succ.html

No matter how successful the attempts to equalize opportunity might be, American society is going to be left with extremely large inequalities.

The famous studies of identical twins raised apart (are) now supplemented by a rapidly growing literature on siblings and half siblings. An analysis bearing directly on The Bell Curve has been conducted at the University of Arizona. It concludes that the greater part of inequality in education and income in the NLSY sibling sample was attributable to genes, with their shared environment playing a subordinate role.

The sibling sample I have used here is limited by specification to youths who grew up in the same home with both biological parents for at least the first seven years of the younger sibling's life. Let us further limit the sample so that we lop off the bottom 25 percent of the income distribution. The results come down to this: In a world where no parents are even close to poverty and all children are born into intact families, we find reduced poverty but income inequality that is not qualitatively different from the levels we now observe. It is hard to see how the existing repertoire of interventions can realistically be expected to narrow income inequality at all.